ENH: Load extension icon from disk

Change extension manager to attempt to load the icon for the extension
from disk, given the URL (which serves to provide a reasonable file
type, hopefully). For now this assumes that the icon already exists;
next, we'll want to try to download it if not.

Note that, rather than using the icon image directly, we render it as a
rounded rectangle in order to apply some rounding to the icon's corners,
for consistency with the web view which does something similar.

Issue 0001907

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23201 3bd1e089-480b-0410-8dfb-8563597acbee